Popcorn is the latest and most unlikely food shown to have health-boosting properties.

The traditional cinema snack contains "surprisingly large" amounts of healthy antioxidant plant chemicals called polyphenols, scientists have found.

Tests showed that the compounds account for 2.5 per cent of popcorn kernels by weight - higher levels than were seen in a range of other cereal products.

Polyphenols, also found in fruits, vegetables, chocolate, wine, coffee and tea, are known to protect the heart and reduce the risk of cancer.

US chemist Dr Joe Vinson, who made the discovery, said: "We really were surprised by the levels of polyphenols we found in popcorn. I guess its because it's not processed. You get all the wonderful ingredients of the corn undiluted and protected by the skin. In my opinion it's a good health food."

Dr Vinson's research supports the idea that polyphenols in whole grains, rather than their fibre content, is what makes them healthy.

His team at the University of Scanton in Pennsylvania analysed a range of whole grain breakfast cereals and snacks.

The researchers found the products contained similar levels of antioxidants per gram as fruits and vegetables.

Popcorn was one of the richest sources. In comparison, breakfast cereals had polyphenol levels ranging from 0.03 per cent to 0.5 per cent by weight.

Huge variations were seen between different brands of the same cereal products due to processing, said Dr Vinson.

"If you made your own granola cereal from grains, nuts and dried fruit, that would be the optimum," he added.

The findings were presented today at the American Chemical Society's annual meeting in Washington DC.

Hot breakfast foods such as porridge oats had disappointingly low levels of polyphenols, said Dr Vinson.